In the medical laboratory technicians and specialists used the term normal level or normal range in the blood to express standard values that are found in healthy people.
Imbalance in the normal level of the standard value of any material under analysis: comes from a defect in the human body and the presence of the results of the analysis of the patient outside its normal level is an imbalance caused by disease or disorder of any kind in the body.
From Wikipedia: In health-related fields, a reference range or reference interval usually describes the variations of a measurement or value in healthy individuals. It is a basis for a physician or other health professional to interpret a set of results for a particular patient.
How normal level or reference ranges is set to measure natural substances in the human body?
Taking a sample from a large group of healthy people in the reconstruction of convergent can find the natural value of the material analyzed.
Other sampling of people in different age groups and different health can be set allowable range and set the levels of disease and treatment and other levels.
From webmed: Each reference range is different because it is created from information from a specific group. For example, the following table shows reference ranges for a sedimentation rate test. This test helps determine whether inflammation, infection, or an autoimmune disease may be present.